i) Field of the Invention
The present invention relates to a navigation system mounted to a vehicle for guiding a vehicle such as a car according to a predetermined route, and more particularly to a vehicle navigation system for orally informing a driver of a route selected.
ii) Description of Related Art
Conventionally, in order to reduce a driver's load, a variety of devices have been proposed and mounted to a vehicle and a navigation system for guiding a vehicle to a destination along a route is known. In this navigation system, when the vehicle passes through a branch point such as an intersection, a selected route is advised to a driver. In an example of a navigation system put to a practical use, an image display device using a display is provided on an instrument panel of the vehicle and a route is informed to the driver.
In addition to the information by use of the image display, another navigation system for issuing route information orally has been known. For example, as disclosed in Japanese Patent Laid-Open No. Sho 62-267900, in a navigation system, when the vehicle has reached a predetermined position (30 m) before an intersection, in order to give route information, an indication is given to the driver orally. In such a navigation system, by using a voice there is no need for the driver to look at a display and he can concentrate on looking ahead.
However, in the conventional navigation system, the point at which a guiding voice is spoken is fixed to a predetermined distance before an intersection. Thus, when moving on a road having many lanes, sometimes, a lane change can not be completed before reaching the intersection and hence a right-hand turn or a left-hand turn can not be carried out.
For example, when the voice speaks "turn to the right at the next intersection" while the vehicle is moving in the leftmost lane, it is necessary to adapt the driving to the flow of surrounding vehicles and to carry out a lane change to the rightmost lane and sometimes it can take a substantial amount of time to effect this lane change. Further, in the case of a road having many lanes, in many cases, a right-hand turn only lane is provided, and when turning to the right, it is required to align the vehicle at the rear of the vehicle row which has formed in the right-hand turn only lane. This has been described in connection with countries where cars drive on the left-hand side of the road. A turning-right only lane of this case corresponds to a turn left only lane of the road in countries where cars drive on the right side, however this is not necessarily so. In this case, it is often necessary to complete the lane change a considerably long time before the intersection. In such a case, the driver needs to know the route change at the intersection earlier.
Further, in the conventional navigation system, the point at which the guide voice is spoken is fixed to the predetermined distance before the relevant intersection and the distance from the intersection is in fact the distance measured from the center of the intersection. Hence, in the case of a crossroad having a wide width in the running direction of a vehicle, a difference between a distance from the center of an intersection and a distance from the entrance of the intersection becomes large. For the driver, the distance to the entrance of the intersection is the necessary information, and, when the navigation is performed based on the central position of the intersection, the vehicle can reach the intersection earlier than he estimated due to the error caused by the aforementioned road width. For example, as shown in FIGS. 1A and 1B, a distance L.sub.C from a vehicle 1 to the center 2 of an intersection is not equal to a distance L.sub.E from the entrance 3 of the intersection and the distance L.sub.E varies depending on a width (in the vehicle running direction) of a crossing road intersecting with a road where the vehicle runs. FIG. 1A illustrates the intersection with a crossroad having a wide width in the running direction of the vehicle and FIG. 1B illustrates the intersection with a crossroad with a narrow width in the running direction of the vehicle. As shown in FIGS. 1A and 1B, even when the distance L.sub.C from the vehicle to the center 2 of the intersection is equal, a distance L.sub.E1 from the vehicle to the entrance 3 of the intersection as shown in FIG. 1A is shorter than a distance L.sub.E2 from the vehicle to the entrance 3 of the intersection as shown in FIG. 1B (L.sub.E1 &lt;L.sub.E2). Hence, when the width of the crossroad is wide, actually, the distance to the entrance 3 of the intersection from the point of voice guidance becomes shorter and thus sometimes the right-hand or left-hand turning chance is missed.
Further, in the conventional navigation system wherein the guiding point is fixed, when the running speed of the vehicle is different, the Lime of reaching the relevant branch point is different. That is, in the case of a fast running speed, the vehicle approaches the intersection relatively quickly and in the case of the low running speed, the vehicle approaches the intersection relatively slowly. Hence, when the vehicle runs at a fast speed, the vehicle can approach the intersection more quickly than the driver expected.
In particular, in the case of carrying out an oral guide of the vehicle, if the voice guidance is not completed, the informing is not substantially executed, and in the conventional navigation system, before the oral guide finishes informing, the vehicle may pass the intersection.